Total Rupture Area Research Articles

Physically Based and Empirical Ground Motion Prediction Equations for Multiple Intensity Measures (PGA, PGV, Ia, FIV3, CII, and Maximum Fourier Acceleration Spectra) on Sakhalin Island

In the present study, empirical attenuation relations for multiple ground motion intensity measures (PGA, PGV, Ia, FIV3, CII, and MFAS) were developed for Sakhalin Island (in the far east of Russia). A recorded strong motion dataset was used, making GMPEs applicable in active crustal regions with an earthquake magnitude range of 4–6 and a distance range of up to 150 km. The hypocentral distance was used as a basic distance metric. For the first time in the research, an analytical representation of Arias intensity (Ia) was obtained in the framework of a multi-asperity source model. Asperities are considered as sub-sources of high-frequency incoherent radiation. The physical representation of the attenuation model in our study was based on a stress drop on the asperities and the ratio of the total rupture area to the combined area of asperities. The average stress drop on asperities for the examined earthquakes was approximately 13.4 MPa, and the ratio of the total rupture area to the asperity area was 0.22, which is generally close to similar estimates for crustal earthquakes. The coefficients and statistical scattering of the attenuation models were also analyzed. Moreover, a magnitude scale based on a modified Arias intensity is proposed in the present study. The new magnitude scale has an explicit physical meaning and is characterized by its simplicity of measurement. It is associated with the acceleration source spectrum level and can be successfully used in early warning systems.

Characterizing Multisubevent Earthquakes Using the Brune Source Model

ABSTRACTAlthough the Brune source model describes earthquake moment release as a single pulse, it is widely used in studies of complex earthquakes with multiple episodes of high moment release (i.e., multiple subevents). In this study, we investigate how corner frequency estimates of earthquakes with multiple subevents are biased if they are based on the Brune source model. By assuming complex sources as a sum of multiple Brune sources, we analyze 1640 source time functions of Mw 5.5–8.0 earthquakes in the seismic source characteristic retrieved from deconvolving teleseismic body waves catalog to estimate the corner frequencies, onset times, and seismic moments of subevents. We identify more subevents for strike-slip earthquakes than dip-slip earthquakes, and the number of resolvable subevents increases with magnitude. We find that earthquake corner frequency correlates best with the corner frequency of the subevent with the highest moment release (i.e., the largest subsevent). This suggests that, when the Brune model is used, the estimated corner frequency and, therefore, the stress drop of a complex earthquake is determined primarily by the largest subevent rather than the total rupture area. Our results imply that, in addition to the simplified assumption of a radial rupture area with a constant rupture velocity, the stress variation of asperities, rather than the average stress change of the whole fault, contributes to the large variance of stress-drop estimates.

The Growth of Earthquake Clusters

Migration of hypocenters is a common attribute of induced injection seismicity and of earthquake swarms, which distinguishes them from aftershock sequences. Spreading of the triggering front is often examined by fitting the time dependence of hypocenter distances from the origin by the pore pressure diffusion model. The earthquake migration patterns however often exhibit not only spreading envelopes but also fast-growing streaks embedded in the overall migration trends. We review the observed migration patterns and show that in the case of earthquake-driven migration, where the new ruptures are triggered at the edge of previous ruptures, it is more suitable to examine the cluster growth as a function of the event index instead of time. We propose a model that relates the speed of seismicity spreading to the average rupture area and the effective magnitude of the hypocenter cluster. Application of the model to selected linearly growing clusters of the 2008 West Bohemia swarm gives an almost linear increase of the measured total rupture area with the event index, which fits the proposed model. This is confirmed by a self-similar scaling of the average rupture area with the effective magnitude for stress drops ranging from 0.1 to 1 MPa. The relatively small stress drop level indicates the presence of voids along the fault plane and a possible role of aseismic deformation.

Parametrizing Physics-Based Earthquake Simulations

Utilizing earthquake source parameter scaling relations, we formulate an extensible slip weakening friction law for quasi-static earthquake simulations. This algorithm is based on the method used to generate fault strengths for a recent earthquake simulator comparison study of the California fault system. Here we focus on the application of this algorithm in the Virtual Quake earthquake simulator. As a case study we probe the effects of the friction law’s parameters on simulated earthquake rates for the UCERF3 California fault model, and present the resulting conditional probabilities for California earthquake scenarios. The new friction model significantly extends the moment magnitude range over which simulated earthquake rates match observed rates in California, as well as substantially improving the agreement between simulated and observed scaling relations for mean slip and total rupture area.

Source study of three moderate size recent earthquakes in the Guerrero seismic gap

We study three recent earthquakes of different types in the Guerrero seismic gap zone with M ≥ 6.5: the 15 July 1996 Guerrero near-coast interplate earthquake (Mw = 6.6), the 18 April 2002 Guerrero near-trench interplate earthquake (Mw = 6.7), and the 11 December 2011 Guerrero normal-faulting inslab earthquake (Mw = 6.5). We compute the slip distributions, estimate source parameters, and model strong ground motions with the finite-fault stochastic method. We use different methods to estimate source parameters in order to observe differences in stress drop and radiated seismic energy among these events. The similarity in seismic magnitude gives us the opportunity to compare our results for the three different types of earthquake and interpret them in terms of the tectonic environments and seismic hazard. We analyze the peak ground accelerations and their relation with the stress drop. We simulated ground motions with the stochastic method. The model parameters are validated against recordings and a stress drop of 3, 15, and 70 MPa is estimated for the near-trench interplate, near-coast interplate, and normal-faulting inslab events, respectively. The near-trench interplate event has the lowest radiated seismic energy and the lowest rupture velocity. This is reflected in the energy to moment ratio of 2.17 × 10−6, 4.52 × 10−6, and 3.96 × 10−6 for the near-trench interplate, near-coast interplate, and normal-faulting inslab events, respectively. We define asperities using two different criteria: (1) based on average displacement and (2) maximum displacement. The asperity area for the near-coast and near-trench interplate events represents about 23 and 24–25 % of the total rupture area, which in the case of the normal-faulting inslab event, it is only 19 and 23 % based on average and maximum displacement, respectively.

Source study of the Jan Mayen transform fault strike-slip earthquakes

Seismic source parameters of oceanic transform zone earthquakes have been relatively poorly studied. Previous studies showed that this type of earthquakes has unique characteristics such as not only the relatively common occurrence of slow events with weak seismic radiation at high frequencies but also the occurrence of some events that have high apparent stress indicating strong high frequency radiation. We studied 5 strike-slip earthquakes in the Jan Mayen fracture zone with magnitudes in the range of 5.9<Mw<6.7. We determined finite-fault slip distributions with the iterative deconvolution method from teleseismic data. The slip models were characterized by determining source parameters on the asperities and background areas. We found that the areas of the asperities represent about 23–28% of the total rupture area with two different criteria based on average slip and maximum displacement. Sensitivity tests were carried out to estimate the variability of slip patterns and source parameters. We observed robust solutions and reliable source parameter estimates from the models. We also determined the radiated seismic energy and apparent stress in order to identify possible slow events. Our results suggest that the analyzed events in Jan Mayen have a regular nature in terms of apparent stress, seismic energy and centroid time delay compared to other oceanic transform fault earthquakes.

Scaling Relationships of Source Parameters of Mw 6.9-8.1 Earthquakes in the Cocos-Rivera-North American Subduction Zone

Research Article| March 25, 2014 Scaling Relationships of Source Parameters of Mw 6.9–8.1 Earthquakes in the Cocos–Rivera–North American Subduction Zone Alejandro Ramírez‐Gaytán; Alejandro Ramírez‐Gaytán aDepartamento de Ciencias Computaciónales, Centro Universitario de Ciencias Exactas e Ingeniería (CUCEI) de la Universidad de Guadalajara, Boulevard Marcelino García Barragán # 1421, CP. 44430, Guadalajara, Jalisco, Méxicol7m8s3r@gmail.com Search for other works by this author on: GSW Google Scholar Jorge Aguirre; Jorge Aguirre bInstituto de Ingeniería, Universidad Nacional Autónoma de México, Ciudad Universitaria, Coyoacán 04510, México DF, Méxicojoagg@pumas.iingen.unam.mxmjaimest@iingen.unam.mx Search for other works by this author on: GSW Google Scholar Miguel A. Jaimes; Miguel A. Jaimes bInstituto de Ingeniería, Universidad Nacional Autónoma de México, Ciudad Universitaria, Coyoacán 04510, México DF, Méxicojoagg@pumas.iingen.unam.mxmjaimest@iingen.unam.mx Search for other works by this author on: GSW Google Scholar Víctor Huérfano Víctor Huérfano cPuerto Rico Seismic Network, Department of Geology, University of Puerto Rico, Residencie 2ª Mayagüez R. 9000, 00680Victor@prsn.uprm.edu Search for other works by this author on: GSW Google Scholar Author and Article Information Alejandro Ramírez‐Gaytán aDepartamento de Ciencias Computaciónales, Centro Universitario de Ciencias Exactas e Ingeniería (CUCEI) de la Universidad de Guadalajara, Boulevard Marcelino García Barragán # 1421, CP. 44430, Guadalajara, Jalisco, Méxicol7m8s3r@gmail.com Jorge Aguirre bInstituto de Ingeniería, Universidad Nacional Autónoma de México, Ciudad Universitaria, Coyoacán 04510, México DF, Méxicojoagg@pumas.iingen.unam.mxmjaimest@iingen.unam.mx Miguel A. Jaimes bInstituto de Ingeniería, Universidad Nacional Autónoma de México, Ciudad Universitaria, Coyoacán 04510, México DF, Méxicojoagg@pumas.iingen.unam.mxmjaimest@iingen.unam.mx Víctor Huérfano cPuerto Rico Seismic Network, Department of Geology, University of Puerto Rico, Residencie 2ª Mayagüez R. 9000, 00680Victor@prsn.uprm.edu Publisher: Seismological Society of America First Online: 14 Jul 2017 Online ISSN: 1943-3573 Print ISSN: 0037-1106 Bulletin of the Seismological Society of America (2014) 104 (2): 840–854. https://doi.org/10.1785/0120130041 Article history First Online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Alejandro Ramírez‐Gaytán, Jorge Aguirre, Miguel A. Jaimes, Víctor Huérfano; Scaling Relationships of Source Parameters of Mw 6.9–8.1 Earthquakes in the Cocos–Rivera–North American Subduction Zone. Bulletin of the Seismological Society of America 2014;; 104 (2): 840–854. doi: https://doi.org/10.1785/0120130041 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search Abstract Seven slip models currently available from kinematic inversions, derived from near‐source strong‐motion and teleseismic body waves in the 0–1.25 Hz frequency range from Mexico’s subduction zone, are used to estimate source‐scaling relationships applicable to the region. Our results are compared with existing scaling relations for subduction environments. The relationships for the rupture area of our results are closer to those of Somerville et al. (2002) than to any other, but, like the others, they have smaller areas than predicted by Somerville et al. (2002). Concerning the combined area of asperities, Murotani et al. (2008) and our results predict smaller areas than those obtained by Somerville et al. (2002). Concerning the area of largest asperity, the relationships obtained in this study are slightly smaller than those described by Somerville et al. (2002); this is a consistent result with the relationships of total rupture area and combined area of asperities. In general, the error estimates for the constrained equations derived in this study in all cases are smaller than those relationships compared here. This might suggest that the expressions obtained in this study could be appropriate for the simulations of strong ground motion for a specific scenario of earthquake slip in the region. Also, these results could be an indication that the relationships vary depending on a specific subduction tectonic region. On the other hand, Aguirre and Irikura (2007) estimated the source area for 31 Mexican earthquakes using corner frequencies; these areas show close resemblance with those predicted by the relationships derived in this study. Based on these findings, an important implication is that two different methodologies to determine the total area of asperities based on either low‐ or high‐frequency data generate similar results.Online Material: Figures of fault models. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Source Scaling of Inland Crustal Earthquake Sequences in Japan Using the S-Wave Coda Spectral Ratio Method

We estimate corner frequencies and stress drops for 298 events ranging from M w 3.2–7.0 in 17 inland crustal earthquake sequences in Japan to investigate the source scaling and variation in stress drops. We obtain the source spectral ratio from observed records by the S-wave coda spectral ratio method. The advantage of using the S-wave coda is in obtaining much more stable source spectral ratios than using direct S-waves. We carefully examine the common shape of the decay of coda envelopes between event pair records. The corner frequency and stress drop are estimated by modeling the observed source spectral ratio with the omega-square source spectral model. We investigate the dependences of stress drops on some tectonic effects such as regionality, focal mechanism, and source depth. The principal findings are as follows: (1) a break in self-similar source scaling is found in our dataset. Events larger than M w 4.5 show larger stress drops than those of smaller events. (2) Stress drops of aftershocks are mostly smaller than those of mainshocks in each sequence. (3) There are no systematic differences between stress drops of events occurring inside and outside the Niigata-Kobe Tectonic Zone in Japan. (4) Clear dependence of the faulting type on stress drops cannot be seen. (5) Stress drops of aftershocks depend on their source depth. (6) The crack size obtained from the corner frequency corresponds to the total rupture area of heterogeneous slip models for large events.

Ometepec-Pinotepa Nacional, Mexico Earthquake of 20 March 2012 (Mw 7.5): A preliminary report

An analysis of local and regional data produced by the shallow, thrust Ometepec-Pinotepa Nacional earthquake (Mw 7.5) of 20 March 2012 shows that it nucleated at 16.254°N 98.531°W, about 5 km offshore at a depth of about 20 km. During the first 4 seconds the slip was relatively small. It was followed by rupture of two patches with large slip, one updip of the hypocenter to the SE and the other downdip to the north. Total rupture area, estimated from inversion of near-source strong-motion recordings, is ~25 km × 60 km. The earthquake was followed by an exceptionally large number of aftershocks. The aftershock area overlaps with that of the 1982 doublet (Mw 7.0, 6.9). However, the seismic moment of the 2012 earthquake is ~3 times the sum of the moments of the doublet, indicating that the gross rupture characteristics of the two earthquake episodes differ. The small-slip area near the hypocenter and large-slip areas of the two patches are characterized by relatively small aftershock activity. A striking, intense, linear NE alignment of the aftershocks is clearly seen. The radiated energy to seismic moment ratios, (Es/M0), of five earthquakes in the region reveal that they are an order of magnitude smaller for near-trench earthquakes than those that occur further downdip (e.g., 2012 and the 1995 Copala earthquakes). The near-trench earthquakes are known to produce low Amax. The available information suggests that the plate interface in the region can be divided in three domains. (1) From the trench to a distance of about 35 km downdip. In this domain M~6 to 7 earthquakes with low values of (Es/M0) occur. These events generate large number of aftershocks. It is not known whether the remaining area on this part of the interface slips aseismically (stable sliding) or is partially locked. (2) From 35 to 100 km from the trench. This domain is seismically coupled where stick-slip sliding occurs, generating large earthquakes. Part of the area is probably conditionally stable. (3) From 100 to 200 km from the trench. In this domain slow slip events (SSE) and nonvolcanic tremors (NVT) have been reported.The earthquake caused severe damage in and near the towns of Ometepec and Pinotepa Nacional. The PGA exceeded 1 g at a soft site in the epicentral region. Observed PGAs on hard sites as a function of distance are in reasonable agreement with the expected ones from ground motion prediction equations derived using data from Mexican interplate earthquakes. The earthquake was strongly felt in Mexico City. PGA at CU, a hard site in the city, was 12 gal. Strong-motion recordings in the city since 1985 demonstrate that PGAs during the 2012 earthquake were not exceptional, and that similar motion occurs about once in three years.

Specifying Initial Stress for Dynamic Heterogeneous Earthquake Source Models

Abstract Dynamic rupture calculations using heterogeneous stress drop that is random and self-similar with a power-law spatial spectrum have great promise of producing realistic ground-motion predictions. We present procedures to specify initial stress for random events with a target rupture length and target magnitude. The stress function is modified in the depth dimension to account for the brittle-ductile transition at the base of the seismogenic zone. Self-similar fluctuations in stress drop are tied in this work to the long-wavelength stress variation that determines rupture length. Heterogeneous stress is related to friction levels in order to relate the model to physical concepts. In a variant of the model, there are high-stress asperities with low background stress. This procedure has a number of advantages: (1) rupture stops naturally, not at artificial barriers; (2) the amplitude of short-wavelength fluctuations of stress drop is not arbitrary: the spectrum is fixed to the long-wavelength fluctuation that determines rupture length; and (3) large stress drop can be confined to asperities occupying a small fraction of the total rupture area, producing slip distributions with enhanced peaks.

Recipe for Predicting Strong Ground Motion from Crustal Earthquake Scenarios

We developed a recipe for predicting strong ground motions based on a characterization of the source model for future crustal earthquakes. From recent developments of waveform inversion of strong motion data used to estimate the rupture process, we have inferred that strong ground motion is primarily related to the slip heterogeneity inside the source rather than average slip in the entire rupture area. Asperities are characterized as regions that have large slip relative to the average slip on the rupture area. The asperity areas, as well as the total rupture area, scale with seismic moment. We determined that the areas of strong motion generation approximately coincide with the asperity areas. Based on the scaling relationships, the deductive source model for the prediction of strong ground motions is characterized by three kinds of parameters: outer, inner, and extra fault parameters. The outer fault parameters are defined as entire rupture area and total seismic moment. The inner fault parameters are defined as slip heterogeneity inside the source, area of asperities, and stress drop on each asperity based on the multiple-asperity model. The pattern of rupture nucleation and termination are the extra fault parameters that are related to geomorphology of active faults. We have examined the validity of the earthquake sources constructed by our recipe by comparing simulated and observed ground motions from recent inland crustal earthquakes, such as the 1995 Kobe and 2005 Fukuoka earthquakes.

Characterization of the Heterogeneous Source Model of Intraslab Earthquakes Toward Strong Ground Motion Prediction

We characterize the heterogeneous source slip model of intraslab earthquakes to compare source scaling properties with those of inland crustal and subduction-zone plate-boundary earthquakes. We extracted rupture area (S), total area of asperity (S a), average slip (D) and average slip on asperity (D a) of eleven intraslab earthquakes following the procedure proposed by Somerville et al. (Seism Res Lett 70:59–80, 1999) and proposed the empirical scaling relationship formula of S, S a, and D for intraslab earthquakes. Under the same seismic moment, an intraslab earthquake has a smaller rupture area and total area of asperity, and smaller average slip than an inland crustal earthquake. The area ratio of asperity area and total rupture area of intraslab earthquakes are similar to those of inland crustal earthquakes. The strong motion generation area (SMGA) scaling of intraslab earthquakes appears self-similar, and those results support the idea the characterized source model of intraslab earthquakes can be modeled in a manner similar to that of inland crustal earthquakes.

Broad, weak regions of the Nankai Megathrust and implications for shallow coseismic slip

Deep within the Nankai Trough subduction zone the plate-boundary thrust slips along a well-imaged megasplay fault system during the megathrust earthquakes that regularly strike southwest Japan. The routing of the active plate-boundary thrust along an upward-branching splay fault causes deep underthrusting of an unusually thick section of material attached to the subducting ocean crust. Here we present three-dimensional seismic reflection data that shows this unusually thick section is fluid-rich sediment that results in broad, weakly-coupled regions of the megathrust down into the updip end of the seismogenic zone. The weakly coupled regions lie above an underthrust low seismic impedance (presumed to be low-velocity and low-density) sediment section that is between one and two kilometers thick and covers ~ 3300 km 2, at least one-eighth of the total rupture area of the 1944 Mw 8.1 Tonankai earthquake. This underthrust sediment section is broader and much deeper than inferred at other margins. Sediment underthrusting into the normally well-coupled seismogenic zone likely releases fluids and elevates fluid pressure, which reduces inter-plate coupling along portions of the megasplay fault and allows coseismic rupture to propagate to unusually shallow depths and generate large tsunami as inferred for the 1944 Tonankai event. Splay faults may be a common, yet transient mechanism for developing weak subduction zone thrusts.

Source Parameters and Tectonic Implications of Aftershocks of the Mw 7.6 Bhuj Earthquake of 26 January 2001

We present and discuss the spatial distribution of more than 1000 aftershocks of the largest continental intraplate earthquake to occur during the modern seismological period. The data were recorded on a network of eight portable digital seismographs deployed for 3 weeks starting 17 days after the mainshock. We have calculated high-quality single-event locations, based on a 1D velocity model determined for the region for earthquakes with magnitudes between ∼2 and 5. Aftershock locations reveal activity concentrated on a nearly east-striking, south-dipping plane, trapezoidal in outline. The active zone tapers from about 45 km along strike at the shallow end, which is about 5 km deep, to no more than 25 km long at a depth of 35 km. The total rupture area was about 1300 km 2 . We estimate the static stress drop of the mainshock at 16 ± 2 MPa. Aftershocks extend nearly through the entire crust, with concentrations in the lower crust at about 26 km and in the upper crust at about 10 km. The fault that ruptured was not mapped at the surface and not known to have been active prior to the 2001 earthquake. The aftershock data are consistent with the Bhuj earthquake resulting from reactivation in contraction of a fault formed under extension within a failed rift.

Differences in ground motion and fault rupture process between the surface and buried rupture earthquakes

We have studied differences in ground motion and fault rupture characteristics between surface rupture and buried rupture earthquakes. We found that the ground motion generated by buried rupture in the period range around 1 second is on average 1.5 times larger than the average empirical relationship. In contrast, ground motion from earthquakes that rupture the surface is 1.5 times smaller in the same period range. This phenomenon is considered to be caused by differences in fault rupture process between the two types of earthquakes. To examine possible reasons of the above effect we analyzed source slip distribution data derived from waveform inversions, and divided them into two groups: surface rupture and buried rupture earthquakes. It was found that the large slip areas (asperities) of surface rupture earthquakes are concentrated in the depth range shallower than about 5 km. In contrast, large slip areas of buried rupture earthquakes are spread over the depth deeper than 5 km. We also found that the total rupture area of buried rupture earthquakes is 1.5 times smaller than that of surface rupture earthquakes having the same seismic moment, and that deep asperities have about 3 times larger effective stress drops and 2 times higher slip velocities than shallow asperities. These observations are verified by numerical simulations using stochastic Green’s function method.

Source Characterization for Broadband Ground-Motion Simulation: Kinematic Heterogeneous Source Model and Strong Motion Generation Area

We estimate strong motion generation areas that reproduce near-source ground motions in a broadband frequency range (0.2-10 Hz) using the empirical Green's function technique. The strong motion generation areas are defined as extended areas with relatively large slip velocities within a total rupture area. Four M 6 class (the 1997 Kagoshima on March and May, 1997 Yamaguchi, and 1998 Iwate) and several moderate-size earthquakes in Japan were analyzed. We examine the relationship between the strong motion generation area and the area of asperities, which is characterized based on heterogeneous slip distributions estimated from low-frequency (<1 Hz) waveform inversions (Somerville et al. , 1999). We performed waveform fitting in acceleration, velocity, and displacement, then obtained the strong motion generation area occupying about a quarter of the total rupture area. The size and position of the strong motion generation area coincide with those of characterized asperities. We find self-similar scaling of seismic moment to both the size of the strong motion generation area and the rise time for a magnitude range of analyzed earthquakes. Based on our results, we propose a characterized source model for the broadband ground-motion simulation, which consists of strong motion generation areas with large slip velocities and a background slip area with a small slip velocity. Waveform modeling of the characterized source model suggests that the strong motion generation areas play important roles in simulating broadband ground motions and have the potential to be an about 10-MPa stress-release area on the fault. Manuscript received 22 August 2002.

Estimation of rupture propagation direction and strong motion generation area from azimuth and distance dependence of source amplitude spectra

Strong motion generation areas which reproduce ground motions in 0.2 to 10Hz were estimated using the empirical Green's function method. This strong motion generation area was somewhat smaller than the total rupture area, and coincident with the area of asperities derived from heterogeneous slip distributions estimated by waveform inversions using lower frequencies (&lt;1Hz). We confirmed that the azimuth and distance dependence of observed source amplitude spectra in the near‐source area, i.e. rupture directivity effects, were controlled by rupture propagation style and size of the strong motion generation area. We found that the source displacement spectra at stations in forward rupture propagation directions had higher corner frequencies and steeper high‐frequency decays, compared with stations in sideways directions. Stations in backward directions had opposite tendencies. Different relationships between size and average corner frequencies of the strong motion generation area were proposed for unilateral and bilateral ruptures with radial propagation.

A time-domain inversion technique for the tempo-spatial distribution of slip on a finite fault plane with applications to recent large earthquakes in the Tibetan Plateau

A time‐domain inversion technique is proposed to invert for the tempo‐spatial distribution of slip on an earthquake fault plane. This technique is based on the idea that a finite fault plane can be divided into several subfaults, each of which can be treated as a point source, and that the source time function (STF) of the finite fault is the weighted sum of the STFs of all the subfaults. This technique is applied to deduce the source processes of three recent large earthquakes in the Qinghai‐Xizang (Tibetan) Plateau. The 1990 April 26 Gonghe, Qinghai, earthquake, MS6.9, ruptured on a fault plane with a strike of 113°, a dip of 68° and a rake of 89°. The fault has a length of 45 km, a width of 15 km and a total moment release of 9.4 × 1018 N m. The rupture initiated at the WNW end of the fault and propagated unilaterally towards the ESE end of the fault. In total, the rupturing lasted for 45 s and was divided into two episodes (the first 9 s long and the second 16 s long with 20 s quiescence in between). There are two distinct rupture regions (nuclei) on the fault plane. These two nuclei, with maximum slips of about 62 and 55 cm, are about 25 km apart in the WNW–ESE direction. The 1996 February 3 Lijiang, Yunnan, earthquake, MS7.0, has a focal mechanism with a strike of 157°, a dip of 48° and a rake of −102°. The earthquake fault is about 45 km with a width of 20 km and a total moment release of 9.8 × 1018 N m. The total duration of the rupture was 16 s, is divided into two episodes (the first 8.5 s long and the second 7.5 s long). The inverted final slip distribution of the Lijiang earthquake also shows two nuclei. One nucleus, with a maximum slip of 40 cm, is located on the NNW portion of the fault at a shallower depth and the other, with a maximum slip of 50 cm, is located on the SSE portion of the fault at a greater depth. The 1997 November 8 Mani, Xizang (Tibet), earthquake, MS7.9, has a focal mechanism with a strike of 250°, a dip of 88° and a rake of 19° and a moment release of 3.4 × 1020 N m. The total duration of the rupture was about 15 s. The inverted final slip distribution of the Mani earthquake shows three nuclei on the fault plane. The first nucleus, with a maximum slip of 956 cm, is located at the WSW end of the fault at 10 km in depth. The second, with a maximum slip of 743 cm, is located at the ENE end of the fault, 55 km away from the WSW end of the fault and at 35 km depth. The third, with a maximum slip of 1060 cm, is about 30 km away from the WSW end of the fault and at about 40 km depth. These three nuclei form a total rupture area of about 70 km in length and 60 km in depth.

Tempo-spatial rupture process of the 1997 Mani, Xizang (Tibet), China earthquake of M S=7.9

An earthquake of M S=7.4 occurred in Mani, Xizang (Tibet), China on November 8, 1997. The moment tensor of this earthquake was inverted using the long period body waveform data from China Digital Seismograph Network (CDSN). The apparent source time functions (ASTFs) were retrieved from P and S waves, respectively, using the deconvolution technique in frequency domain, and the tempo-spatial rupture process on the fault plane was imaged by inverting the azimuth dependent ASTFs from different stations. The result of the moment tensor inversion indicates that the P and T axes of earthquake-generating stress field were nearly horizontal, with the P axis in the NNE direction (29°), the T axis in the SEE direction (122°) and that the NEE-SWW striking nodal plane and NNW-SSE striking nodal plane are mainly left-lateral and right-lateral strike-slip, respectively; that this earthquake had a scalar seismic moment of 3.4×1020 N·m, and a moment magnitude of M W=7.6. Taking the aftershock distribution into account, we proposed that the earthquake rupture occurred in the fault plane with the strike of 250°, the dip of 88° and the rake of 19°. On the basis of the result of the moment tensor inversion, the theoretical seismograms were synthesized, and then the ASTFs were retrieved by deconvoving the synthetic seismograms from the observed seismograms. The ASTFs retrieved from the P and S waves of different stations identically suggested that this earthquake was of a simple time history, whose ASTF can be approximated with a sine function with the half period of about 10 s. Inverting the azimuth dependent ASTFs from P and S waveforms led to the image showing the tempo-spatial distribution of the rupture on the fault plane. From the "remembering" snap-shots, the rupture initiated at the western end of the fault, and then propagated eastward and downward, indicating an overall unilateral rupture. However, the slip distribution is non-uniform, being made up of three sub-areas, one in the western end, about 10 km deep ("western area"); another about 55 km away from the western end and about 35 km deep ("eastern area"); the third about 30 km away from the western end and around 40 km deep ("central area"). The total rupture area was around 70 km long and 60 km wide. From the "forgetting" snap-shots, the rupturing appeared quite complex, with the slip occurring in different position at different time, and the earthquake being of the characteristics of "healing pulse". Another point we have to stress is that the locations in which the rupture initiated and terminated were not where the main rupture took place. Eventually, the static slip distribution was calculated, and the largest slip values of the three sub-areas were 956 cm, 743 cm and 1 060 cm, for the western, eastern and central areas, respectively. From the slip distribution, the rupture mainly distributed in the fault about 70 km eastern to the epicenter; from the aftershock distribution, however, the aftershocks were very sparse in the west to the epicenter while densely clustered in the east to the epicenter. It indicated that the Mani M S=7.9 earthquake was resulted from the nearly eastward extension of the NEE-SWW to nearly E-W striking fault in the northwestern Tibetan plateau.

TheMw= 8.0 Antofagasta (northern Chile) earthquake of 30 July 1995: A precursor to the end of the large 1877 gap

AbstractAn Mw = 8.0 earthquake occurred on 30 July 1995 in the Antofagasta region (northern Chile). The main rupture, corresponding to thrust faulting, developed from 10 to 50 km in depth along the subduction interface between the Nazca and the South American plates. The 1995 earthquake took place just south of the large seismic gap where a great earthquake (M = 9) had occurred in 1877. Most of the 1995 rupture was located within a local network consisting of nine short-period stations that had been previously installed at the southern end of the 1877 gap, and the aftershock sequence could be accurately monitored. Little destruction resulted from the 1995 earthquake in spite of its large size. Ground acceleration in Antofagasta reached 29% of gravity. A tsunami wave, 2 to 2.5 m high, was observed along the coast from Mejillones to Taltal. One strong foreshock (Mw = 6.2) occurred in the 1995 hypocentral region 6 months before the main event.Body-wave modeling of broadband seismograms from the global network, along with the analysis of the aftershock distribution, allows us to propose a well-constrained model for the whole rupture process. Some additional details of the rupture were obtained from an accelerometer record at Antofagasta. The main rupture started as a double even with thrust mechanism below the southern part of the Mejillones peninsula, and it propagated southward in a N200°E direction with an average velocity of 2.8 km/sec. It ended near the trench in normal faulting. The total rupture area and seismic moment are 185 × 90 km2 and 1.2 × 1028 dyne-cm, respectively. The aftershock distribution delineates a well-defined rupture surface along the subduction interface. The distribution of epicenters during the first 20 h of aftershock activity shows a sharp northern boundary beneath the Mejillones peninsula. Hence, the 1995 main rupture did not propagate north of the Mejillones peninsula into the 1877 gap. Aftershocks during the following 2 weeks indicate a growth of the initial rupture zone toward the north. The mechanisms of the strongest aftershocks are similar to that of the mainshock. The down-dip termination of the main rupture corresponds to the maximum depth (50 km) of the region that had been identified as the locked part of the subduction interface from the analysis of the microseismicity recorded by the local network prior to the 1995 event.A well-constrained dislocation model is proposed for the 1995 main rupture, which produces surface displacements in good agreement with available observations of coastal uplift and GPS measurements. The dislocation model, as well as Global Positioning System (GPS) measurements, indicate that the 1995 earthquake generated E-W extension in the coastal region of Antofagasta. The Atacama fault, located 40 to 50 km above the 1995 main rupture, showed small fresh surface ruptures near Sierra Remiendos (70 km to the SSE of Antofagasta) with a maximum vertical offset of 20 cm. This offset corresponds to normal faulting, which is in agreement with the E-W co-seismic extension.The Mejillones peninsula appears to be the surface expression of a barrier that interrupted the propagation of the 1995 rupture to the north into the region of the 1877 gap. Modeling of static stress changes induced by the Antofagasta earthquake indicates an increase in compressive stresses along a direction transverse to the trench immediately to the north of the 1995 rupture surface. Thus, the chances for the reactivation of the 1877 gap after this event are greater now.